Conventionally there have been various systems for reducing the noise contained in the video signal. For instance, one system extracts the noise components from the input video signal and subtracts the noise components extracted from the current video signal so as to be able to reduce the noise. The noise components can be extracted from for example non-correlation component between adjacent horizontal scan lines of the video signal, a field non-correlation component or a frame non-correlation component. The noise reduction apparatus of this nature may be configured in a recursive type or a non-recursive type.
Recently, a digital processing of the picture information has been researched in the aspect of requirement of high picture quality. When the picture is digitized, the data amount of the picture becomes huge. Thus, it is necessary to compress the data for recording and transmitting the data. As the compression method is a high efficiency encoding system that adapts a transform encoding such as a DCT (discrete cosine transform) encoding which has a high compression efficiency and a low main current is currently the compression technique. In the high efficiency encoding system, an input picture signal is divided into blocks each comprised of 8.times.8 pixels, so as to implement a DCT processing on the DCT blocks on such block by block basis.
This system adapts not only an intra-frame compression which carries out compressions within each frame by the DCT processing but also an inter-frame compression for reducing the redundancy in a time-axis direction using the inter-frame correlation. The inter-frame compression is used to calculate a frame difference between the two successive frames and implement DCT processing for the differential value using the nature that the general motion pictures of two successive frames are similar. Particularly, a motion compensated inter-frame predictive encoding for reducing the prediction error by predicting the picture motion and calculating the inter-frame difference is available.
In such a compression apparatus noises caused by a quantization (quantization noise) are also generated. As a solution, Japanese Patent Application; Tokkai-Hei 4-88795 proposes an apparatus for reducing noises by specifying noise reduction characteristics suitable for a luminance signal and a chrominance signal respectively. Japanese Patent Application; Tokkai-Hei 5-227431 proposes methods for detecting and reducing a mosquito noise which occurs in decoding of the encoded data using DCT. Japanese Patent Application; Tokkai-Hei 4-88795 proposes a decoder in which noise reduction parameters are compared to each other for the chrominance signal and the luminance signal.
Further, Japanese Patent Application; Tokkai-Sho 61-288678 proposes a method for reducing the quantization noise by a suitable prediction vector information at the predictive encoding. Japanese Patent Application; Tokkai-Hei 6-224773 proposes a method for generating noise reduction coefficients responding to quantized outputs and the results of the picture motion detection.
FIG. 15 is a block diagram showing the conventional noise reduction apparatus which is suitable for the encoder and the decoder of the motion picture. FIG. 16 is a block diagram showing a detailed arrangement of an encoder/decoder 39 shown in FIG. 15. An encoder/decoder 4 shown in FIG. 16 is a combination of the devices which are proposed in Japanese Patent Applications; Tokkai-Sho 61-288678 and Tokkai-Hei 6-224773.
A picture signal input through an input terminal 1 is applied to a memory 2 so as to be memorized. From the memory 2, a block of picture data, for example, comprised of 8 pixels along a horizontal scan line.times.8 lines along in the vertical direction read-out and applied to the encoder/decoder 4 via a subtracter 3. The encoder/decoder 4 encodes the input picture data by the DCT processing, quantization processing and the variable length encode processing and outputs the encoded data through an output terminal 11. Further, the encoder/decoder 4 restores the original picture by decoding the encoded data and outputs the decoded data to the adder 5.
The picture data from the memory 2 is also applied to a motion detector 9. As mentioned above, the motion detection is implemented in the predictive encoding. To the motion detector 9, a picture data of the former frame is applied for example, from a memory 8. The motion detector 9 detects the motion of the picture on a block by block basis by controlling the reading-out of memories 2 and 8 in cooperation with a memory controller 10, so as to provide a motion vector to a memory controller 11 and the encoder/decoder 4. In the inter-frame compression mode, the memory controller 11 controls the reading-out of the memories 2 and 8 based on the motion vector.
It is assumed that the intra-frame compression mode is designated. In this case, a switch 6 selects terminal b and applies "0" to the subtracter 3. Thus, the picture data of the current frame (current signal) is applied to the encoder/decoder 4. The current signal is encoded in the encoder/decoder 4 and then outputted from an output terminal 7. And, the encoder/decoder 4 decodes the encoded data and applies it to an adder 5. To the adder 5, "0" is applied from the switch 6, and the adder applies the decoded picture data to the memory 8 as it is.
It is assumed that the inter-frame compression mode is designated. In this case, the switch 6 selects a terminal a and provides the contents of the memory 8 to the subtracter 3. The memory 8 wherein the decoded picture data of the former picture is memorized provides the stored block picture data at the blocking position based on the motion vector as the motion compensated reference picture data. The subtracter 3 carries out a subtraction between the current signal and the motion compensated reference data so as to calculate the prediction error. The encoder/decoder 4 encodes the prediction error from the subtracter 3 so as to output the encoded data via the output terminal 7.
The encoder/decoder 4 decodes the encoded data so as to apply the decoded data to the adder 5. In this case, the reconstituted data of the prediction error is applied to the adder 5. The adder 5 adds the motion compensated reference picture data from the memory 8 to the prediction error from the encoder/decoder 4 so as to restore the original picture and provides the restored original picture to the memory 8. Thus, the reference picture data to be used in the next encoding operation is stored in the memory 8.
Hereinafter, the encoding of the intra-frame compression mode and the inter-frame compression mode is implemented by the same operation repeatedly.
The operation of the encoder/decoder 4 will be explained in reference to the FIG. 16.
The current signal from the subtracter 3 is applied to a DCT unit 15 of the encoder/decoder 4. The DCT unit 15 converts the input signal from the spatial coordinate axis elements to a frequency component by a two-dimensional DCT processing for a 8.times.8 pixel block. Accordingly, it is possible to reduce the spatial correlation component. That is, the conversion coefficient output from the DCT unit 15 is applied to a quantizer 16, where the conversion coefficient is quantized with a predetermined quantization range. As a result, the redundancy degree of one block signal is reduced. Here, the quantization range of the quantizer 16 is controlled by a rate controller 21.
The quantized data from the quantizer 16 is applied to a noise reduction (hereinafter referred to NR) unit 17 by a zigzag scan from the horizontal low and vertical lower area toward a higher area in each block. The NR unit 17, as disclosed in the Japanese Patent Application; Tokkai-Hei 6-224773, reduces the noise of the quantized output based on the inter-frame non-correlation component. The inter-frame non-correlation component is containing the original non-correlation component of the signal and the noise component. For instance, the non-correlation component is none (zero) in the freeze-frame picture, and the inter-frame non-correlation component is the noise component. The NR unit 17 determines the original non-correlation component of the signal by the motion vector, and enlarges the NR coefficient as the noise component is large. The NR unit 17 multiplies the inter-frame correlation component by the NR coefficient and subtracts the result of the multiplication from the quantized output so as to reduce the noise.
The output from the NR unit 17 is applied to a variable length coder (hereinafter referred as VLC) 18. The VLC 18 outputs the encoded data by the Huffman encode for the quantized output based on the predetermined variable length code table such as a Huffman code table for instance. Here, in the Huffman encoding, a combination data of the number of "0" succession in the quantized output last (zero run-length) and the bit number of the non-zero coefficient is encoded. Thus, the short bit is assigned to the data which have a high appearance probability, and the long bit is assigned to the data having the low appearance probability. As a result, the transmission data amount will be reduced further more.
The encoded output from the VLC 18 is applied to a buffer 19. The buffer 19 which is constructed by a first-in first-out memory, outputs the input encoded data to the output terminal 7 with a predetermined rate. The generating rate of the encoded output from the VLC 18 is the variable rate. The buffer 19 accommodates the difference between the generating rate of the encoded data and the transmission rate of the transmission channel. Here, the output from the VLC 18 is applied to a rate controller 21 also. The rate controller 21 controls the quantization range of the quantizer 18 based on the amount of the codes generated from the VLC 18 so as to control the generating rate of the encoded data.
The output from the NR unit 17 is also applied to an inverse-quantizer 22 to make the reference picture. The quantized output from the NR unit 17 is inverse-quantized in the inverse-quantizer 22, and implemented the inverse-DCT processing in an inverse-DCT unit 23, so as to restore the original picture data. Here, as mentioned above, the output from the inverse-DCT unit 23 is also the prediction error in case of that the output of the subtracter 13 is the prediction error. The output of the inverse-DCT unit 23 is applied to a non-linear circuit (hereinafter referred to NL) unit 24.
The NL unit 24 , as disclosed in Japanese Patent Application; Tokkai-Sho 61-288678, eliminates the quantization distortion (quantization noise) caused by the quantization processing. The NL unit 24 improves the S/N by controlling its output when the level of the inter-frame non-correlation component is small, and reduces the over-shoot on the edge portion by controlling its output when the level is large. The NL unit 24 determines the level of the inter-frame non-correlation component by the motion vector, and controls the non-linear characteristics based on the motion vector, so as to reduce the noise by corresponding with various kinds of pictures. For example, it is possible to reduce the noise clinging on a flat part and that of the freeze part inside of the block containing the movable edge part appropriately. The output of the NL unit 24 is applied to the adder 5.
As mentioned above, in the encoder/decoder 4 as shown in FIG. 16, the noise caused mainly by the current signal is reduced in the NR unit 17 and the noise caused mainly by the quantization, it means the prediction error is reduced in the NL unit 24. As a result, the encoded data which is reduced the noise is obtained.
However, the NR unit 17 and the NL unit 24 determines the levels of the inter-frame correlation components by the motion vectors, and the parameters for deciding its characteristics are controlled on the motion vector on detection by detection basis. That is, the parameter for reducing the noise is changed on block by block basis. As a result, though the noise caused by the predicted distortion and the current signal is reduced, there will be differences of the noise reduction effect on block by block basis. So, there was a drawback that a flickering caused by the differences of the noise reduction effect appears on the picture.
As described above, in the conventional noise reduction apparatus mentioned above had a problem that the noise reduction parameter is controlled by the motion detecting on block by block basis, and the noise reduction effects of every block are different. As a result, the flickering in block appears on the picture.